Photophysiological effects of Fe concentration gradients on diatom-dominated phytoplankton assemblages in the Antarctic Peninsula region

Abstract

The Southern Ocean is generally a high-nutrient and low-chlorophyll (HNLC) area, primarily due to the limitation of micronutrients, such as iron (Fe). Nevertheless, microphytoplankton may bloom if Fe is available and if light and mixing conditions are suitable. This study aimed to investigate the ecological and photophysiological responses of the phytoplankton, such as size, community composition, nutrient usage and photosynthetic potential, under high irradiance levels and over an iron concentration gradient. A 6-day deck incubation experiment was conducted during the austral summer of 2013, using phytoplankton populations from the upper mixed layer in the Bransfield Strait waters. After sampling the water (Fe clean protocol), experimental units were maintained in on-deck incubation in triplicate for 6days with 3 treatments (Control: natural sea water; +Fe: Fe addition; +DFA: Fe less available). +Fe treatment samples were inoculated with FeCl2 (1nM), while in the +DFA treatment, a Fe-binding agent (desferrioxamine) was added which reduces Fe availability tenfold. Values of chlorophyll a, phytoplankton biovolume, PSII maximum quantum efficiency (Fv/Fm) and the relative electron transport rate (rETR) were measured in all treatments. After six days of experimentation, chlorophyll a in the +DFA treatment increased by a lower proportion (6.4mgm−3, compared with the +Fe treatment, which increased to 10.2mgm−3) and presented Fv/Fm values of 0.55±0.02 (compared with the +Fe treatment, with a value of 0.62±0.02). Microscopic analysis showed the preponderance of larger diatoms in the +Fe treatment, while smaller diatoms dominated in the +DFA, in particular the Pseudo-nitzschia species. Although desferrioxamine might have inhibited biogenic Fe uptake in the +DFA treatment, phytoplankton continued to grow, however with lower responses within the measured physiological parameters, indicating the first signs of Fe limitation. Considerable phytoplankton growth, represented by a doubling of chlorophyll a concentration and a massive biovolume increase after 6days in all three treatments, indicates that the waters of the study region in the Bransfield Strait were not limited by the microelement Fe.